The present invention involves a method of testing a pipeline cut, particularly but not exclusively in or for an underwater or below the seabed pipeline, and particularly using time reflectometry techniques such as time domain reflectometry (TDR).
There is a strict legal framework of national, regional and international regulations which govern how operators decommission offshore facilities. Abandonment of wells and the decommissioning of offshore platforms are big challenges for the oil and gas industry that require considerable resources and investment in executing the work in an environmentally safe and economical way. It is estimated that around 120-150 platforms and 600-1000 wells are abandoned in the Gulf of Mexico alone each year and this figure continues to rise as other fields reach the decommissioning phase.
A typical platform decommissioning programme consists of the following activities, which may span a period of 1-4 years depending on the size of the platform and its location:
Well isolation
Well conductor removal
Plant cleaning
Preparation for platform removal
Platform removal
Conductor removal is an important and challenging phase in the decommissioning programme due to the significant number and lengths of the conductors to be removed.
There are several ways to cut and remove conductor casings once the wells have been abandoned. Typically explosive devices or abrasive water jet cutting (AWJC) tools are used to sever the conductor approximately 5 m (or 15 ft) below the sea bed or mud line, and the casings are then removed with heavy lift equipment. Operators tend to prefer the use of AWJC tools on drilling casings, as explosive cutting has higher safety concerns, and causes the cut point to flare out, which can lead to complications (and possibly the need for further cutting by ROV/divers) when removing the string through the conductor guides.
Naturally, it is desired to ensure that the cut through the conductor has been successful, but confirming the success of a through cut of the conductor string by the methods mentioned above is also problematic. When explosive cutting is used, the conductor string physically ‘drops’ as proof of a successful cut. However with AWJC tools, the process is more complicated and requires a top tension to be applied to the conductor during the cutting process. This requires a shock load-compensated jack that has to pull load of up to 300 tons to apply the top tension to the casing during the cutting process, but the jack must be able also allow immediate release once the casing under tension is cut free. This system is very equipment intensive, costly and may take some time to install and operate as the jack also has to overcome the initial soil friction on the outer casing. As advances in the AWJC systems move towards ‘rigless’ designs (i.e. with no drilling derrick) the use of top tensioned jacks is becoming less compatible with such systems.